The goal of this project is to dissect the age-related changes in immune cells that affect responses to microbial vaccination in the elderly. Aging is associated with a decline in the ability to combat infections and is linked with at least two immune alterations: immunosenescence, or the functional decline of the immune system over time; and inflammaging, a chronic inflammation status believed to trigger major age-related chronic diseases. These alterations may explain insufficient responses to microbial vaccines in the elderly, leaving this population highly vulnerable to the consequences of serious infections like S. pneumoniae, the focus of the current study. Indeed, S. pneumoniae causes significant morbidity and mortality in the elderly, and the two currently approved pneumococcal vaccines-PPSV23 (Pneumovax(r)) and PCV13 (Prevnar(r))-have been used with mixed success in this population. For example, while PPSV23 is efficacious in healthy young adults, several studies have shown decreased efficacy in elderly. There, it prevents pneumococcal bacteremia in elderly, but does not reduce the risk of Community-Acquired Pneumonia (CAP). PCV13 was recently reported to reduce by ~50% vaccine-type pneumococcal, bacteremic and non-bacteremic CAP, but still left a significant subset of individuals underprotected. The underlying reasons for the suboptimal efficacy of these two vaccines is unclear. It is possible that they elict qualitatively and quantitatively different immune responses, but this remains to be established. Furthermore, no definitive conclusions can be drawn as to their comparative efficacy in the elderly, since in all studies in elderly cohorts each vaccine was compared to placebo and not each other. Thus, many questions as to the nature of (and variability in) vaccine responsiveness remain unanswered. Our hypothesis is that the altered immune responses of the elderly to pneumococcal vaccines are caused by genomic alterations associated with aging that result in dysfunction in one or more of three immune cell types-antigen presenting cells (APCs), follicular helper T cells (Tfh) and B cells-critical for adaptive immunity. We propose three Specific Aims to test our hypothesis - Aim 1: To vaccinate healthy elderly with two distinct pneumococcal vaccines, collect longitudinal blood samples and assess pneumococcal-specific antibody responses; Aim 2: To establish the transcriptional and epigenetic profiles of elderly blood immune cells linked with antibody responses to pneumococcal vaccination; and Aim 3: To examine the functional status of immune cells in the elderly stratified according to their pneumococcal vaccine responder status. Our approach integrates flow cytometry and other functional assessments with cutting-edge genomic approaches, and is designed to yield in-depth phenotypic, transcriptional, epigenetic and functional data for these three key immune cell types. These efforts are poised to yield unprecedented large-scale datasets and knowledge concerning immune responses in healthy elderly.